Do I correctly understand the reported problem here?

set.add(0) correctly raises TypeError in each case, but:

(1) the exception message changes between versions;

(2) and also changes depending on whether or not sys.trace is active.

I don't think the exception message (as opposed to the exception type) is part of the public API. This means it can change without warning, at any time, so long as it is sensible and human readable.

Unless there is something documenting the precise wording used in this case, guaranteeing that it will be kept stable, I don't think this should be considered a regression or a bug. Do you disagree?